The present invention relates to a light signal transmission/reception system, and more particularly to a light signal transmission/reception system suitable for transmitting a transmission signal with a high precision.
Generally, in order to avoid various adverse effects caused by noises in signal transmission, an optical data transmission using as a transmission line an optical fiber cable has been adopted as a countermeasure for such noises.
In a light transmission, however, the amount of light emission from a light emission diode for converting an electrical signal into a light signal is under the influence of temperature as shown in FIG. 1. In particular, even if the current flowing into the light emission diode is maintained constant, the amount of light emission is decreased with the temperature rise. In order to compensate for this, a method shown in FIG. 2 has been adopted heretofore. This compensation method will briefly be described.
An electrical analog signal applied to a terminal 1 is inputted to a driver circuit 2 of a transmitter 6. The driver circuit 2 adjusts the amplitude of the analog signal, and the adjusted signal is outputted to a light emission diode 3. Ordinarily, the light emission diode 3 is used in such a region as the input electrical signal and the light emission amount of the light emission diode 3 become proportionate with each other. In this manner, the light emission diode 3 converts the input electrical signal into a light signal. The converted light signal is transmitted through an optical fiber cable 5 to a receiver 10.
In the receiver 10, the input light signal is converted at a light reception diode 7 into an electrical signal. The level of the electrical signal is converted at an amplifier circuit 8 to output an electrical analog signal at a terminal 9.
However, in this case, the light emission diode 3 in the transmitter 6 is under the influence of temperature, as shown in FIG. 1. Thus, in order to compensate for such influence, a portion of the light signal transmitted from the light emission diode 3 to the optical fiber cable 5 is received at a monitoring light reception diode 4 to feedback the received light signal to the driver circuit 2, thereby controlling the driver signal to the driver circuit 2. In particular, as the light emission amount decreases with the temperature rise, an electrical signal or current supplied to the light emission diode 3 from the driver circuit 2 is rendered to increase owing to the above feedback effect, so that the light emission amount of the light emission diode 3 can always be maintained constant.
If there is a temperature difference between the transmitter 6 and receiver 10, the difference however can not be compensated. The signal transmission apparatus, disclosed in the Official Gazette of Japanese Patent Application Laid-open No. 56-54136, is an apparatus of the type that the temperature characteristics of the light emission and reception elements in the respective transmitter and receiver are retained equal to each other. Also in this case, if the temperatures of environmental atmospheres at the locations of the transmitter and receiver are different from each other, the error in the transmitted signal caused by the temperature difference can not be compensated.
Alternatively, among those apparatuses as used at the location where the atmospheric temperature is subjected to an abrupt change, there is known in the art a signal transmission apparatus as disclosed in an Official Gazette of Japanese Patent Application Laid-open No. 56-12143. In the apparatus, a signal to be transmitted is frequency-converted and a first frequency signal having as a reference the center frequency of the frequency-converted signal is transmitted via an optical fiber to the receiver. In addition, a second frequency signal which is the center frequency of the reference is transmitted via another optical fiber to the receiver. In this case, two light emission elements and two light reception elements respectively connected to the two different optical fibers must have the same temperature characteristics. However, light emission and reception elements with the same temperature characteristics require very cumbersome processes in manufacturing.